Certain carcinogens damage cells by binding to DNA. Enzymatic removal of the promutagenic alkylation lesion 06-methylguanine from DNA has been achieved in vitro in normal human tissues, cultured normal human bronchial epithelial cells (NHBE) and fibroblasts (HBF). This demonstrates the catalytic activity of 06alkylguanine-DNA alkyltransferase (06-MT). Alkyltransferase activity has been detected in all human tissues tested. Repair of the spontaneous uracil-DNA lesion, which is associated with Bloom's syndrome and repaired by uracil-DNA glycosylase (UDG), has also been demonstrated in vitro in human tissues and cells. Repeated exposure to alkylating agents induces higher levels of 06-MT, i.e., adaptive responses, in some cells. In our previous studies using NHBE cells, 06-MT was not induced by N-methyl-N-nitrosourea (MNNU); aldehydes inhibited DNA repair by both 06-MT and enhanced MNNU mutagenicity in HBF. To resolve the indications of these results, we are studying; a) in vitro measure of 06-MT and UDG in broncho-alveolar lavage cells (BALCs) and peripheral blood mononuclear (PBM) cells from smokers and nonsmokers, and b) the in vitro effects of cigarette smoke condensate (CSC) on cultured NHBE cells, lung tumor cells (HuT-292) and DNA repair enzymes. Preliminary results show: large inter- and intra-individual variations in enzyme levels that are smaller in smokers than in nonsmokers; smokers and cotinine-positive donors with overall lower enzyme levels and higher adduct levels in PBMs (but not in BALCS) than nonsmokers and cotinine-negative donors; higher 06-MT levels in adduct-containing PBMs and BALCs than in those without adducts, but lower increases in cotinine-positive donors as compared to cotinine-negative donors.